In every new generation, the further development of portable appliances is distinguished by new functionalities being provided. Every new functionality in this case potentially contributes to an increase in the power consumption. For the user of these appliances, it is becoming increasingly more important to be informed about the remaining time for which they can be used, that is to say the remaining battery or rechargeable-battery capacity. In particular, nickel-cadmium and nickel-metal-hydride rechargeable batteries (NiCd and NiMH rechargeable batteries) are increasingly being replaced by rechargeable batteries which are based on lithium (Li), because these have an excellent energy density per unit volume.
The battery voltage or rechargeable-battery voltage can be measured easily and at low cost as a measure of the state of charge, but does not comply with stringent requirements for the desired accuracy. For example, the rechargeable-battery voltage decreases continuously during discharging, but the relationship between the voltage level and the remaining charge varies highly non-linearly with the temperature and the discharge rate. Depending on the type of rechargeable battery, a voltage measurement as a measure of the state of charge of the rechargeable battery can lead to a measurement error of 100%, or considerably more than this. This can admittedly be corrected well if the temperature and the discharge rate are known.
However, the measurement of these additional parameters makes the process more complicated and expensive than a charge measurement.
A charge measurement which can be carried out by means of a current measurement and a time measurement is therefore more accurate, because current is defined as the change in the charge per unit time. In order to allow a very accurate estimate of the remaining battery capacity to be made, there is a developing trend towards charge meters (fuel gauges) which track the net current flow into and out of the battery or rechargeable battery.
A charge meter such as this comprises an integrated circuit (IC) with a computation unit and additional external components for evaluating the current measurement and for determining the state of charge. A further integrated circuit with external circuitry is, on the other hand, required for charging of the rechargeable battery, by means of which the charging process can be controlled such that it takes place quickly and without any damage to the rechargeable battery.
The publication “DS2770 Battery Monitor and Charge Controller” from Dallas Semiconductor proposes an integrated circuit with external circuitry that combines the functions both of charging and of detection of the state of charge. In particular, this saves valuable space on a circuit board.
The IC can be used to set two rechargeable-battery types (lithium or NiMH) with different charging characteristics. A defined charging program is carried out for the respective rechargeable-battery type depending on its threshold voltage being reached for undervoltage at the output connections of the rechargeable battery.
The charging process itself is carried out with the aid of two constant-current sources, which the IC drives as a function of a voltage criterion. If the rechargeable-battery voltage is below the threshold voltage, that is to say the rechargeable battery has been deep-discharged, a first external transistor with a current-limiting resistor produces a low constant charging current until the threshold voltage is reached. During a normal charging process, when the rechargeable-battery voltage reaches or is higher than the threshold voltage, a comparatively high constant current is fed into the rechargeable battery, by means of a second external transistor. The IC switches between the first and the second transistor, and thus between the low and high charging current, as required. Depending on the rechargeable-battery type, the charging process is ended when a time criterion or a temperature criterion is reached.
The second function of the circuit, that is to say measurement of the rechargeable-battery current and its direction, is carried out by means of the voltage drop across a measurement resistor (shunt) in the ground-side branch of the charging circuit. The current direction into or out of the rechargeable battery is established by a subtraction unit, which establishes the mathematical sign and the magnitude of the current flow. The values are digitized, and registers are used to store the instantaneous current flow and the current accumulated over time.
The two functions of the IC are provided separately. In particular, the rechargeable battery is charged simply by switching the external constant-current sources on and off. If this IC is operated with a charging source with a low impedance, external current limiting is required, by means of additional, and therefore space-consuming, components.